It is known that uranium oxides are suitable as oxidation catalysts for a series of complete and selective oxidations. A typical example of the use of uranium-based catalysts is the CO oxidation to CO2, as has been described, for example, by Campbell et al. in J. Molec. Cat. A: Chem., (2006), 245(1-2), 62-68. Further oxidations catalysed by uranium-containing mixed oxides are, for example, that of isobutene to acrolein (Corberan et al., Ind. Eng. Chem. Prod. Res. Dev., (1984), 24, 546, and 1985, 24, 62) and that of propylene to acrolein and acrylonitrile (U.S. Pat. No. 3,308,151 and U.S. Pat. No. 3,198,750). In addition, the total oxidation of VOCs (volatile organic compounds) over U3O8 is also known, which has been studied especially by Hutchings et al. (Nature, (1996), 384, p 341). Uranium oxide as a support for nanoscale gold particles is described, for example, in Green Chemistry (2005), 7(11), 768-770 and (2007) 9(3), 267-272. Applications in the field of hydrogen chloride oxidation are not disclosed in this connection.
As is common knowledge, ruthenium is used especially as a reduction catalyst or as an oxidation catalyst (Handbook of Heterogeneous Catalysis, 1997, p. 2160 and p. 2181).
An oxidation under more severe conditions with regard to temperature and partial oxygen pressure is the process, developed by Deacon in 1868, of catalytic hydrogen chloride oxidation with oxygen:4HCl+O22Cl2+2H2O.
The oxidation of hydrogen chloride to chlorine is an equilibrium reaction. The equilibrium position shifts away from the desired end product with increasing temperature. It is therefore advantageous to use catalysts with maximum activity which allow the reaction to proceed at low temperature.
The first catalysts for hydrogen chloride oxidation comprising the catalytically active constituent of ruthenium were described as early as 1965 in DE 1 567 788, in this case proceeding from RuCl3. The support claimed was Al2O3 and SiO2. The activity of this combination is relatively low, since these supports cannot provide any lattice oxygen atoms for the oxidation process.
Further Ru-based catalysts with ruthenium oxide or mixed ruthenium oxide as active constituents have been claimed in DE-A 197 48 299. The content of ruthenium oxide is 0.1% by weight to 20% by weight and the mean particle diameter of ruthenium oxide 1.0 nm to 10.0 nm.
Further Ru catalysts supported on titanium oxide or zirconium oxide are known from DE-A 197 34 412. For the preparation of the ruthenium chloride catalysts described therein, which comprise at least one compound from titanium dioxide and zirconium dioxide, a series of Ru starting compounds was specified, for example ruthenium-carbonyl complexes, ruthenium salts of inorganic acids, ruthenium-nitrosyl complexes, ruthenium-amine complexes, ruthenium complexes of organic amines or ruthenium-acetylacetonate complexes. In a preferred embodiment, titanium dioxide in the form of rutile was used as the support. Although Ru catalysts have quite a high activity, they tend to undergo sintering and hence deactivation at relatively high temperatures. To increase the economic viability, however, a further enhancement of the activity combined with good long-term stability is needed.
The supported ruthenium oxidation catalysts developed to date have insufficient activity or stability for hydrogen chloride oxidation. Although it is possible to enhance the activity by increasing the reaction temperature, this leads to sintering/deactivation or to the loss of the catalytically active constituent.
DE 1 078 100 discloses catalysts comprising salts or oxides of silver, uranium or thorium, which are present on inert supports composed of kaolin, silica gel, kieselguhr or pumice. It is not disclosed that the resulting catalysts are calcined, as a result of which a low stability of the catalysts disclosed is to be expected. Moreover, it is not disclosed that the support alone can have a catalytic activity and can consist of uranium oxide. What is disclosed is always a composition which requires the presence of silver and salts or oxides of rare earths. Accordingly, for the lack of further disclosure, it can be assumed that the technical teaching is aimed at a cocatalytic effect which enables a conversion only in an interaction of the individual catalytically active constituents.
This is disadvantageous because both the use of silver and of the salts or oxides of the rare earths lead to the catalyst being economically disadvantageous compared to alternatives without these constituents. Especially the use of silver can be considered to be particularly disadvantageous here in view of the continuously rising costs of this noble metal.
It is thus an object of the present invention to provide a catalyst which accomplishes the oxidation of hydrogen chloride with high activity and/or stability while being obtainable in an economically advantageous manner compared to the prior art.